Why cast out demons?
Why cast out demons? They don't leave automatically!
It's vital to understand the importance of casting out demons. Some believers assume that demons will automatically leave on their own accord once the legal grounds are removed, but that simply isn't true. I believe there are many times when they do, simply because the demon chooses not to hang around where he has no 'hold' or 'right' to a person, but that is by no means true in every case.
Removing legal grounds isn't always enough
If demons left automatically when their legal grounds were removed, why didn't Jesus simply tell those who were possessed with demons that their sins were forgiven, as He did with others (Matthew 9:2)? Instead He rebuked demons and commanded them to come out and they often made a big scene when they came out! Wouldn't it be easier to tell them that their sins were forgiven? One time Jesus even said, "For whether is easier, to say, Thy sins be forgiven thee; or to say, Arise, and walk?" (Matthew 9:5) If you ask me, it would be a LOT easier to say "thy sins be forgiven", than to rebuke demons and have them throw the person to the ground in convulsions, foam at the mouth, scream to the top of a person's lungs, etc. There's a clear purpose in casting out demons, and Jesus' life here on earth was an example for us to learn from (John 14:12, "He that believeth on me, the works that I do shall he do also."), and a large part of His ministry was casting out demons.
We may be cleansed from legal grounds in our lives, but unwelcome guests (evil spirits) may still need to be told to leave. Let's say that you lived in an apartment and had a roommate, but one day you chose to have the place to yourself, so you started paying the full rent for your apartment (so you have full legal rights to live there alone), but where your problem lies is that your stubborn roommate (who no longer has a legal right to live there) refuses to leave. The same principal applies in the spiritual realm when dealing with demons. This is why Jesus gave us authority over demon spirits to cast them out. If demons left on their own accord once their legal grounds are broken, there would be no need to cast them out! But Jesus made it clear that them who believe will be casting out demons (Mark 16:17).
Mark 9:29 also makes it clear that some demons, regardless if their legal grounds are removed or not, require prayer and fasting to drive out, "And he said unto them, This kind can come forth by nothing, but by prayer and fasting." Jesus was perfectly capable of forgiving a person's sins when He was here on earth (Luke 7:47), so for Him to tell us that some demons require prayer and fasting to drive out, tells us that demons simply don't leave the moment their legal grounds are removed.
The nature of demons
It's vital to learn of the nature of demons, because when you realize that they are stubborn and rebellious beings that often need to be TOLD to leave, you'll understand why it's important to cast them out. Let's take a look at some of the tantrums they threw as Jesus cast them out:
In Mark 9:20, we find a story of when a demon was so upset that he had to leave, that he took control over the boy and threw him into a convulsion with foam coming out of his mouth, "And they brought him unto him: and when he saw him, straightway the spirit tore him (took control over the boy and threw him into a convulsion); and he fell on the ground, and wallowed (rolled around) foaming (foam was coming out of his mouth)."
In Luke 9:39, we find another story where a demon threw an awful tantrum as he was driven out, "And, lo, a spirit taketh him, and he suddenly crieth out; and it teareth him (threw the person into a convulsion) that he foameth again, and bruising him hardly (harshly bruising him physically) departeth from him."
Perhaps one of the most common manifestations mentioned in the Bible is found in Mark 1:26, "And when the unclean spirit had torn him, and cried with a loud voice, he came out of him." Properly interpreted, the word cried here actually refers to screaming. The demon didn't go out quietly, it went out screaming and thew the boy into a convulsion on it's way out!
If demons throw such fits as JESUS Himself cast them out, what makes us think they will peacefully leave on their own accord? What amazes me, is how people expect demons to play nice and leave automatically, but they would never expect a criminal to willfully give up his lifestyle without the authority of policemen stepping in and forcing the man to either change or be cast into prison.
Deliverance was not available in the OT
Back in the OT, once a person picked up a demon, they had to pretty much live with it. The casting out of demons only came on the scene when Jesus arrived on earth and had the authority to cast them out.
In Mark 1:27, we find that the people around Jesus were shocked when they saw Him exercise His authority over evil spirits because they had never seen this happen before, nor was it ever mentioned in all of history, "And they were all amazed, insomuch that they questioned among themselves saying, What thing is this? what new doctrine is this? for with authority commandeth he even the unclean spirits, and they do obey him."
Purpose of authority
In NT Greek, the authority to cast out demons translates back to the Greek word exousia, and means, "From G1832 (in the sense of ability); privilege, that is, (subjectively) force, capacity, competency, freedom, or (objectively) mastery (concretely magistrate, superhuman, potentate, token of control), delegated influence: - authority, jurisdiction, liberty, power, right, strength."
To have authority means to have the ability to force another person (or spirit) to subject itself to you and obey you. Our authority is accessed by faith, as Jesus gave His disciples the authority to cast out demons (Luke 9:1, Luke 10:17), but in Matthew 17:14-21, they were unable to cast out a particular kind of demon that was apparently much stronger then usual, simply because they lacked faith (see verses 19-20). They didn't lack authority; they lacked the faith that was necessary to exercise their authority.
How did Jesus use His authority to cast out demons? By speaking fourth a command, or as Matthew 8:16 tells us, by His Word He drove out demon spirits, "When the even was come, they brought unto him many that were possessed with devils: and he cast out the spirits with his word, and healed all that were sick:"
With authority, Jesus drove demons out of people, as we can see in Mark 9:25, "When Jesus saw that the people came running together, he rebuked the foul spirit, saying unto him, Thou dumb and deaf spirit, I charge thee, come out of him, and enter no more into him."
With authority, those in the early church were also casting out evil spirits, as we can see in Acts 16:18, "But Paul, being grieved, turned and said to the spirit, I command thee in the name of Jesus Christ to come out of her. And he came out the same hour."
This same authority belongs to you and I as believers, as we are seated with Christ in the heavenly realm (Ephesians 2:6), and as Jesus clearly pointed out, those who believe can cast out demons in Mark 16:17, "And these signs shall follow them that believe; In my name shall they cast out devils."
A misinterpreted verse exposed
I've seen ministers use Acts 8:5-7 as so-called proof that demons are automatically expelled from people, simply because it doesn't say 'as clearly' who drove them out, such as "Paul rebuked the spirit..." If we take a close look at this verse, I believe it becomes very clear that it was Philip who was casting the spirits out:
Acts 8:5-7, "Then Philip went down to the city of Samaria, and preached Christ unto them. And the people with one accord gave heed unto those things which Philip spake, hearing and seeing the miracles WHICH HE DID. For unclean spirits, crying with loud voice, came out of many that were possessed with them: and many taken with palsies, and that were lame, were healed."
People were listening to Philip teach and taking heed because they saw the miracles that HE was doing. People were being healed, demons were being driven out, etc. Those demons didn't just decide to come out on their own, no more than people were 'auto-healed'. If such a verse could be used to argue that demons auto-shed from people when they accept Christ, then it could also be used to prove that when people 'truly' accept Jesus (they say that those who have demons haven't *truly* accepted Jesus), they will be physically healed as well. Oh the power of twisted scriptures!! :)